Some radio frequency (RF) communication systems employ constant-envelope modulation schemes, such as frequency modulation (FM) or constant phase modulation (CPM), thereby allowing power amplifiers and transmitters to work in the non-linear operating zone near saturation. This permits the power efficiency of the amplifiers to be increased, without generating intermodulation products in adjacent channels. However, as the popularity of RF mobile communications has increased, the availability of spectrum has become limited. To more efficiently utilize available spectrum, other types of modulation, which are more spectrally efficient, have been developed, such as n-QPSK (quadrature phase shift keying) and n-QAM (quadrature amplitude modulation) with baseband filtering. Unfortunately, these more efficient modulation techniques do not feature constant envelopes and they are more likely to create spectral disturbances due to intermodulation products.
One way to avoid interference caused by intermodulation products, is to permit RF power amplifiers to operate in their linear range. However, this leads to a significant decrease in the power efficiency of the amplifiers.
To overcome these distortion and efficiency problems, predistortion circuits have been developed for adaptively linearizing power amplifiers so that they may operate in the non-linear range. FIG. 1 is a graphical representation 10 of a typical power amplifier magnitude characteristic showing output compression. As the input level to the amplifier increases, the operation of the amplifier becomes non-linear. In the non-linear range, the amplifier output becomes compressed. To make up for the compression, i.e., linearize the amplifier, more drive power descriptively termed a “push” increment is added to the input level to achieve the desired linear output (dashed line) in the non-linear range. Essentially, the amplifier can be driven in a linear fashion by combining a predistortion signal with the original input level.
U.S. Pat. No. 5,049,832 by Cavers describes a predistortion approach for adaptively linearizing a power amplifier over its operational range. The predistorted drive signal of the Cavers approach is generated by multiplying a complex modulated input signal by complex gain coefficients stored in an adaptively determined look-up table (LUT). In its digital form, the predistorted drive signal is a composite signal consisting of an input component along with an inverse distortion signal. This composite signal is applied to appropriate digital-to-analog converters (DACs) for conversion to baseband analog signals before being applied to the power amplifier. In the predistortion system of the Cavers patent, the entire predistorted drive signal is processed through one set of I/Q DACs. This limits the wide offset signal-to-noise ratio at the system output to a level directly determined by the bit resolution of the I/Q DACs. Although this may be acceptable in some communication systems, in other systems, this limitation could negatively affect the overall system performance.
An alternative approach that has been used to achieve lower noise at wide offsets involves placing cavity filters at the outputs of the transmitter power amplifiers. In some circumstances, this approach has been highly beneficial. However, cavity filters are relatively expensive, and they generally reduce transmitter output power and add to the packaging volume of transmitters.
Accordingly, there is a need for an adaptive digital predistortion system that can be used to linearize RF power amplifiers and reduce off-channel noise caused by the limited precision of DACs and analog to digital converters (ADCs).